Add ostruct to gemspec dependencies

[padde]

Fixes #121

This gem was historically included in the default gemset. Starting with Ruby 3.3.5 though, requiring ostruct without explicitly adding it as dependency generates the following warning:

warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.
Also please contact the author of grape-swagger-rails-0.5.0 to request adding ostruct into its gemspec.

[dblock]

@padde Thanks! Will you please add Ruby 3.3 to CI and make sure it passes? https://github.com/ruby-grape/grape-swagger-rails/blob/master/.github/workflows/test.yml

[grape-bot]

	1 Warning
⚠️	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

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📖	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

Here's an example of a CHANGELOG.md entry:

* [#122](https://github.com/ruby-grape/grape-swagger-rails/pull/122): Add ostruct to gemspec dependencies - [@padde](https://github.com/padde).

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[padde]

Thanks @dblock

I think a lot of specs are failing here because with Rails 7, calling warn on the singleton instance of ActiveSupport::Deprecation is, well... deprecated. See rails/rails#47354

I will first work on that.

[dblock]

Thanks, lmk if you need help. Let's have a matrix with various rails versions eventually too.

[padde]

For reference: #124

[padde]

@dblock good point about the CI matrix, I did that as well but in a separate PR: #125

[padde]

And finally, this PR will then fix compatibility with the upcoming Ruby 3.5 release: #126